Microporous copper film and electroless copper plating solution for obtaining the same

ABSTRACT

A printed circuit board comprising a metal copper film having 10 5  to 10 9  micropores per square centimeter wherein the metal copper film is prepared by dipping a plating object into an electroless copper plating solution comprising a copper ion, a complexing agent, a hypophosphorus acid compound, a metallic catalyst for initiating the reductive reaction, and a compound containing an acetylenic bond.

This application is a 371 of PCT/JP98/00689 (Feb. 19, 1998).

TECHNICAL FIELD

The present invention relates to a microporous metal copper film, andmore particularly, to a metal copper film having a vast number ofmicropores of a micron unit and to an electroless copper platingsolution capable of obtaining this copper film. The present inventionalso relates to a plating product provided with this metal copper film.

BACKGROUND ART

A multi-layer printed circuit board is conventionally manufactured byfirst preparing a copper clad laminate for the inner layer by processinga copper foil on the copper clad laminate to form a printed circuit;then subjecting the above copper foil to a surface roughening treatment(generally comprising degreasing, followed by a soft etching process asexemplified by treatment with ammonium persulfate, sodium persulfate,cupric chloride, sulfuric acid-hydrogen peroxide system and the like, aswell as an activating treatment); subsequently building an acicular filmof copper oxide or cuprous oxide on top of the foil by a process such asblackening or browning; and bonding a copper clad laminate for the outerlayer or copper foils in multiple layers with a material impregnatedwith a thermosetting resin (i.e. a “prepreg”) to fabricate a multi-layerlaminated board having a high adhesion strength.

Since electric continuity has to be established to each layer of themulti-layer laminated board manufactured in the above process, athrough-hole plating on holes drilled through the board is required.However, the conventional method has had a drawback whereby penetrationof the acid solution used in the catalyst treatment process for platingthrough-holes or penetration of the plating solution in the electrolesscopper plating process tends to dissolve the film made from copper oxideor cuprous oxide, thereby causing a phenomenon called “pink ring”(i.e.“haloing”).

On the other hand, there is an alternative method in which a printedcircuit is formed on a copper clad laminate using a copper foil that ispre-processed by surface roughening to eliminate the need for surfaceroughening as well as the oxide film forming processes required in themethod described above, thereby providing a multi-layer printed circuitboard. This method, however, has shortcomings such as inferior patternresolution for the printed etching resist or the etching resist forultra-violet exposure, which are associated with the surface roughnessof the copper foil.

In order to correct the above shortcomings, the present inventors haverecently developed a method for forming a uniform and acicular copperfilm with excellent adhesion strength using electroless copper plating(Japanese Patent Application Laid-Open No. 116176/1992 and InternationalPatent Application No. PCT/JP96/03829). This technology enabledmanufacturing of a copper clad laminate having a copper film with a highadhesion strength, without the aforementioned shortcomings.

DISCLOSURE OF THE INVENTION

While earnestly making research efforts to improve on the technologydescribed above, the present inventors have discovered the fact that,rather than a uniform and acicular copper film, a microporous copperfilm can be produced depending on the surfactant used, thereby providinga copper clad laminate having a copper film with high adhesion strength.

Such a microporous copper film is not yet known at present. The presentinventors have further found that not only this microporous copper filmcan be utilized for the copper clad laminate but also this film itselfcan be used as a metal filter or a catalyst or its carrier. Thesefindings have led to the completion of the present invention.

Accordingly, it is an object of the present invention to provide a metalcopper film having one hundred thousand to one billion micropores perone square centimeter.

Another object of the present invention is to provide an electrolesscopper plating solution comprising a copper ion, a complexing agent, ahypophosphorous acid compound as a reducing agent, and a metalliccatalyst for initiating the reductive reaction, characterized by furthercomprising a compound containing an acetylenic bond.

Yet another object of the present invention is to provide a platingproduct having a microporous copper film produced by dipping a platingobject into the above electroless copper plating solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph (magnification of 5,000) of a crystal structureshowing the outward appearance of the electroless copper film of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

As examples of the compound containing an acetylenic bond which isformulated in the electroless copper plating solution of the presentinvention, compounds represented by the following formula (I) are given:

wherein R₁ and R₂ respectively represent an alkyl group and R₃ and R₄respectively represent a hydrogen atom or a lower alkyl group.

Specific examples of the compound containing an acetylenic bond includealkyne diols such as 2,4,7,9-tetramethyl-5-decyne-4,7-diol,3,6-dimethyl-4-octyne-3,6-diol, and the like. The compounds containingan acetylenic bond are commercially available under the trademarkSurfinol 104 (manufactured by Nisshin Chemical Industries Co., Ltd.).

Except for the compound containing an acetylenic bond as describedabove, the electroless copper plating solution in the present inventioncan be prepared from known materials for an electroless copper platingsolution that uses a hypophosphorous acid compound as a reducing agent.For example, a copper ion for electroless copper plating can be obtainedfrom ordinary copper salts such as copper sulfate, cupric chloride, orcopper nitrate; and for the complexing agent, any compound that cancomplex the above copper ions, such as citric acid, tartaric acid, malicacid, EDTA, Quadrol, or glycine, can be used.

For the hypophosphorous acid compound as a reducing agent, compoundssuch as hypophosphorous acid or sodium hypophosphite can be mentioned.As the metallic catalyst for initiating the reductive reaction, metalssuch as nickel, cobalt, or palladium can be used in the form ofinorganic salts.

Referring to each ingredient of the electroless copper plating solutionin the present invention, if nickel is used as the metallic catalyst forinitiating the reductive reaction, it is preferable to have a copper ionconcentration of 0.007 to 0.160 mol/l and a nickel ion concentration of0.001 to 0.023mol/l, where the desirable mol ratio between copper andnickel ions is approximately at 13:1.

It is preferable to use a complexing agent in an amount of 1 to 10 timesthe amount of copper ions by mol ratio. Also, it is preferable toformulate 0.1 to 1.0 mol/l of a hypophosphorous acid compound as areducing agent.

In the event that a metal other than nickel is used as the metalliccatalyst for initiating the reductive reaction, the quantity and ratiogiven above may be applied while the most suitable quantities can bedetermined separately by experiments.

The electroless copper plating solution in the present invention, inaddition to ingredients as described above, may be formulated withvarious other ingredients as appropriate. One such other ingredient is abuffer agent for conditioning the solution pH.

It is noted here that an embodiment may be possible where theelectroless copper plating solution in the present invention is preparedas a concentrated composition for dilution to several times or more by adiluent such as water at the time of application.

The electroless copper plating in the present invention can be performedusing the electroless copper plating solution of the present inventionprepared as described above, and in accordance with conventional platingprocedures. In performing these procedures, it is preferable to removedissolved oxygen beforehand from the electroless copper platingsolution, and to this end, blowing an inert gas such as nitrogen orargon through the solution prior to commencing the plating process ispreferred.

Also, it is preferable that the temperature of the electroless copperplating solution for electroless copper plating in the present inventionbe 40 to 100° C., and that the plating time be 5 minutes or longer.Further, for the electroless copper plating in the present invention,although it is preferable to use vibrational stirring to preventunnecessary oxidation of the solution, an inert gas may also be used tosimultaneously perform stirring and deoxidizing. Furthermore, it ispreferable to control the pH within the range of 8 to 10 in theelectroless copper plating in the present invention.

The electroless copper film deposited from the above electroless copperplating solution has an outward appearance shown in FIG. 1. The numberof pores ranges from 10⁵ to 10⁹ per square centimeter and generally from3×10⁶ to 3×10⁸ per square centimeter. Also, the diameter of microporesranges from 0.01 to 100 μm and generally from 0.1 to 10 μm.

Such a copper film having a vast number of micropores has greatsignificance in that this is a novel material which has not beenconventionally known and that the film can be produced in a chemicallysimple method.

This copper film has excellent adhesion strength resulting from theimpregnation of a vast number of micropores with a prepreg. In addition,various applications of the copper film are under consideration in viewof the vast number of micropores.

For example, the copper film is allowed to deposit on a smooth glassplate or a plastic plate and then peeled off to produce a copper foilhaving a vast number of micropores, which can be utilized as a filter.Furthermore, a material produced by depositing an adequate metalincluding a precious metal such as rhodium or another metal such asnickel, on such a copper foil may be used as a catalyst.

In the electroless copper plating method of the present invention, it ispossible to add an acetylenic bond-containing surfactant disclosed inJapanese Patent Application Laid-open No. 116176/1992, e.g. Surfinol 465(manufactured by Nisshin Chemical Industries Co., Ltd.) to theelectroless copper plating solution to obtain a copper film having avast number of micropores formed by small acicular crystals grown overthe entire surface.

Other features of the invention will become apparent in the course ofthe following description of the exemplary embodiments which are givenfor illustration of the invention and are not intended to be limitingthereof.

EXAMPLE 1

Preparation of electroless copper plating solution (1):

An electroless copper plating solution was prepared based on thecomposition given below, according to the conventional method:

(Composition) Copper sulfate  0.032 mol/l Sodium citrate  0.052 mol/1Sodium hypophosphite  0.270 mol/l Boric acid  0.500 mol/l Nickel sulfate0.0024 mol/l Surfinol 104 1.0 g/l pH 9.0 *Manufactured by NisshinChemical Industries Co., Ltd.

Using the above electroless copper plating solution, electroless copperplating was performed on a copper clad laminate for the inner layer(FR-4; epoxy resin) at 60° C. for 30 minutes. The resulting copper filmwas examined with a scanning electron microscope to confirm theformation of micropores as shown in FIG. 1.

EXAMPLE 2

Strength of adhesion to a resin substrate:

The strength of adhesion of the microporous copper film of the presentinvention to various resin substrates was evaluated in terms of the peelstrength of a multi-layer board which was prepared by adhering thecopper film to the resin substrates through a prepreg after electrolesscopper plating using the composition of Example 1.

As a result, the adhesion strength in case of FR-4 was 1.2 kgf/cm, andthe adhesion strength in case of a BT-800 resin (bismaleimide triazine)was 0.7 kgf/cm. These results were higher than in case of a blackeningtreatment. In case of a PPE-S resin (polyphenylene ether), the adhesionstrength was 0.2 kgf/cm when the microporous copper film of the presentinvention was provided, whereas little adhesion strength can be obtainedwhen a blackening treatment was carried out.

As is clear from the above results, the microporous copper film of thepresent invention is effective for inner layer copper foil treatmentespecially for recent resin substrates having high heat resistance,electric reliability, chemical resistance, and the like.

EXAMPLE 3

Preparation of electroless copper plating solution (2):

An electroless copper plating solution was prepared based on thecomposition given below, according to the conventional method:

(Composition) Copper sulfate  0.032 mol/l Sodium citrate  0.052 mol/lSodium hypophosphite  0.270 mol/l Boric acid  0.500 mol/l Nickel sulfate0.0024 mol/l Surfinol 104* 1.0 g/l Surfinol 465* 0.1 g/l pH 9.0*Manufactured by Nisshin Chemical Industries Co., Ltd.

Electroless copper plating was performed on a copper clad laminate forthe inner layer (FR-4) in the same manner as in Example 1. The adhesionstrength evaluated was 1.3 kgf/cm. The resulting copper film wasexamined with a scanning electron microscope to confirm the formation ofsmall acicular crystals grown over the entire surface including theinner surface of the micropores.

Industrial Applicability

The microporous copper film of the present invention is depositedbetween a base copper foil and each of various resin substrates wherebya high adhesion strength can be obtained. Various applications areanticipated thanks to the microporous characteristics.

The microporous copper film of the present invention may be utilized,for example, as a metal microfilter or a catalyst or its carrier.

What is claimed is:
 1. A printed circuit board comprising a metal copperfilm having 10⁵ to 10⁹ micropores per square centimeter wherein themetal copper film is prepared by dipping a plating object into anelectroless copper plating solution comprising a copper ion, acomplexing agent, a hypophosphorous acid compound, a metallic catalystfor initiating the reductive reaction, and a compound containing anacetylenic bond, wherein the compound containing an acetylenic bond isrepresented by the formula (I):

wherein R₁ and R₂ respectively represent an alkyl group and R₃ and R₄respectively represent a hydrogen atom or a lower alkyl group.
 2. Anelectroless copper plating solution comprising a copper ion, acomplexing agent, a hypophosphorous acid compound as a reducing agent,and a metallic catalyst for initiating the reductive reaction, furthercomprising a compound containing an acetylenic bond, wherein thecompound containing an acetylenic bond is represented by the formula(I):

wherein R₁ and R₂ respectively represent an alkyl group, and R₃ and R₄respectively represent a hydrogen atom or a lower alkyl group.
 3. Anelectroless plating method comprising: dipping a plating object into anelectroless copper plating solution comprising a copper ion, acomplexing agent, a hypophosphorous acid compound, a metallic catalystfor initiating the reductive reaction, and a compound containing anacetylenic bond to deposit a microporous copper film, wherein thecompound containing an acetylenic bond is represented by the formula(I):

wherein R₁ and R₂ respectively represent an alkyl group and R₃ and R₄respectively represent a hydrogen atom or a lower alkyl group.
 4. Aprinted circuit board comprising a plating product having a microporouscopper film which is prepared by dipping a plating object into anelectroless copper plating solution comprising a copper ion, acomplexing agent, a hypophosphorous acid compound, a metallic catalystfor initiating the reductive reaction, and a compound containing anacetylenic bond, wherein the compound containing an acetylenic bond isrepresented by the formula (I):

wherein R₁ and R₂ respectively represent an alkyl group and R₃ and R₄respectively represent a hydrogen atom or a lower alkyl group.